The present invention generally relates to magneto-optical recording of information on a magneto-optical disk and more particularly to a recording head for recording and/or erasing an information signal on and from a magneto-optical disk by a finely focused optical beam while applying a predetermined magnetic field across the disk.
Generally, a magneto-optical recording and reproducing apparatus records an information on a magneto-optical disk by a recording head which irradiates a finely focused optical beam on a recording surface of the disk to cause a local heating of a magnetic recording medium on the recording surface of the disk while applying a predetermined magnetic field across the disk. When erasing the information already recorded on the disk, on the other hand, the direction of the magnetic field thus established has to be reversed. The recording head having a capability of such an inversion of the magnetic field will be referred to hereinafter a magneto-optical recording head or simply a recording head. As a means for producing such a reversible magnetic field, a permanent magnet or an electromagnet is used commonly, wherein permanent magnet is preferred from the view point of reducing the size of the recording head. When a permanent magnet is used for this purpose, a mechanism for turning the magnet such that the S-pole and the N-pole of the magnet are selectively faced to the recording surface of the disk is needed for changing the direction of the magnetic field.
FIG. 1 shows a typical prior art mechanism for applying such a biasing magnetic field across the magneto-optical disk for the purpose of recording and further for erasing. Referring to the drawing, a permanent magnet bar 2 held rotatably by a bracket 3 is disposed in a vicinity of a magneto-optical disk 1 such that the bar 2 extends in a radial direction of the disk 1 and the the magnet bar 2 is rotated by a motor 5 via a reduction gear system 4. The permanent magnet bar 2 carries the S-pole on its one side and the N-pole on the other side and is turned by 180 degrees when the operational mode of the magneto-optical disk, apparatus is switched between a recording mode and an erasing mode. In one example, the S-pole is brought to face a recording surface of the disk 1 in the recording mode and the N-pole is brought to face the recording surface of the disk 1 in the erasing mode.
In such a prior art head, there is a problem in that a complex construction such as the bracket 3 and the transmission gear 4 is needed and the cost of the head is increased. As a result of the complexity of the mechanism using a number of parts, there is a problem that the parallelism between the magnet bar 2 and the recording surface of the disk 1 tends to be lost unless the parts used in the mechanism is manufactured with high precision. When the parallelism is lost, the distance between the magnet and the disk is changed at the time of recording and at the time of erasing and strength of the magnetic field tends to be changed accordingly. Such a change of the magnetic field may cause poor recording or insufficient erasing. However, such a high manufacturing precision as required for the prior art recording head increases the cost of the head further. Further, as a result of use of the transmission gear system 4, the turning over of the magnet bar 2 is slow and produces noise during its operation.
When the magnet bar 2 is mounted directly on a rotary shaft of a motor 5, on the other hand, there is a problem in that the magnet bar 2 is tend to be mounted with offset with respect to the real center of the rotary shaft and the magnet bar 2 is rotated with eccentricity. When such an eccentricity occurs in the rotation of the magnet bar 2, the strength of the magnetic field is changed at the time of recording and at the time of erasing similarly to the foregoing case. In order to eliminate such an eccentricity, a delicate and time-consuming adjusting procedure is necessary.